The invention relates to a method of detecting foreign bodies in continuous mass streams of fibrous material, strand-like material or material similar to bulk goods with the aid of microwaves, in which the mass flow is led through the field of a microwave resonator and in which the change (A) in the resonant frequency, effected by the material, and the change (B) in the width of the resonance curve of the microwave resonator is determined.
In numerous processes in the processing industry, in which nonmetallic products are processed in mass streams, foreign bodies which have unintentionally got into the product stream normally have an extremely damaging effect on the following process steps, the quality of the product or the production plant.
For example, during the production of yarns in the textile industry, good homogeneity in the preparation for spinning is already important if the cotton and plastic fibers are aligned in parallel and homogenized in carding and drawing devices. Inhomogeneities in the fiber distribution, caused by incorporated foreign bodies or neps, have an effect in the final product, the finished woven material or the curtain, as a reduction in quality which is obvious to any customer. It is therefore a matter of outputting a warning in good time, by measuring with the aid of microwave resonators, as early as in the spinning preparation machine and, if appropriate, of arranging for the foreign body to be removed manually or automatically.
In the production of cigarettes, the bales of tobacco supplied from the tobacco growing regions are sometimes contaminated with all kinds of foreign bodies, such as textile residues, pieces of plastic, rubber parts, metal wires, timber residues or insects. In addition, on the long paths from tobacco preparation to the cigarette machine, foreign bodies can get into the tobacco as a result of inattention. Ultimately, a foreign body which has not been eliminated from a cigarette not only means an extreme reduction in the quality of the cigarette but, depending on the type of the foreign body during the combustion and inhalation of the smoke, a hazard to the consumer if a halogen acid can be produced, such as during the combustion of halohydrocarbons. It is therefore a matter, by means of reliable detection of foreign bodies at a point where foreign bodies can no longer get into the tobacco (for example during the formation of the cigarette rod and wrapping it with paper) with the aid of a microwave resonator after the rod has been cut up into pieces of rod, of arranging for the contaminated cigarette to be removed automatically. Since these removed cigarettes can no longer be recycled but are burned, a high requirement is placed on the measurement technique, for economic reasons: the erroneous rejection rate with still reliable foreign body detection may not exceed one per thousand of the cigarettes produced since, even at this rate, in the case of about 200 billion cigarettes typically produced per company per year, 200 million would be destroyed as contaminated.
In many branches of the chemical industry, undesired foreign bodies can lead not only to a reduction in the product quality but also to damage to the production plants. Cellulose acetate synthetic fibers (mainly used for the production of cigarette filters) are produced from cellulose acetate granules dissolved in acetone by being pressed out of fine spinning nozzles.
Foreign bodies in the granules, which do not go into solution, can block up the spinning nozzles and lead to fiber breakages during the fiber production. Glass is produced from molten quartz sand. Foreign bodies in the quartz sand can lead to irreparable damage to the melting furnaces. Here, too, it is a matter, by means of the product management of the bulk material by means of a suitable measuring arrangement, of detecting the changed material composition owing to a foreign body and of arranging for the latter to be removed without the enormous density fluctuations, which occur at the same time in the pouring process having a detrimental effect on the measurement.
Foreign bodies are therefore to be understood firstly to mean all critical changes in the physical or chemical composition of a product which go beyond the normal statistical fluctuation of the composition of the product stream around a typical average at the measurement point. This also includes metallic contaminants. Secondly, however, in the case of a constant composition, this can also be understood to mean large mass flow fluctuations, for example extreme density minima as a result of voids in the cigarette or maxima as a result of excessive proportions of ribs or material thickenings in the textile strand being formed.
Detection methods for individual specific types of foreign bodies have been known for a long time, such as inductive metal detectors (e.g. DE 3714009 A1, Schroder, Hauni, Hamburg 1987). However, these only act on a specific type of foreign bodies, have an effect which is only severely restricted and on most foreign bodies no effect at all.
More general foreign body detection methods have primarily been disclosed in the tobacco industry in the last decade. Attempts have been made to utilize the changed flow behavior of some foreign bodies to remove them by means of specific air swirling (e.g. U.S. Pat. No. 5,267,576 Heitmann, Hauni, Hamburg 1992) or (WO 00/40105, Rizzolo, Fabriques de Tabac Reunies, Neuchatel, 1998). In this case, however, the only limited efficiency of these plants has been shown in practice. A specific proportion of primarily coarse foreign bodies is certainly removed as a function of its geometric shape, but the generally finely cut, fibrous foreign bodies are not registered.
Many attempts have been made to detect foreign bodies with the aid of optical detectors, preferably infrared detectors, and to eliminate them by blowing them out with air. (U.S. Pat. No. 4,657,144, Martin, Philip Morris, New York, 1985) or (U.S. Pat. No. 5,476,108, Dominguez, R. J. Reynolds Tobacco Company, Winston Salem, 1992). Since optical or IR sensors can barely penetrate the surface of the product being investigated, however, the product must be spread out to form a very thin layer in order to achieve optical detection of the foreign bodies (DE 4325838, Roether, Hauni, Hamburg, 1993). For this reason, these methods can also be used only at the positions where such spreading out of the material flow is possible, that is to say for example not on the finished tobacco rod covered with paper. In this case, however, similarities in the surface and color between foreign body and product stream lead to many foreign bodies not being detected. In addition, spreading out the main stream of the product to be examined at the high throughput rates which are generally normal in industrial processes leads to swirling of the product, so that an optically detected foreign body also changes its position because of the swirling and cannot be blown out.
It is known to evaluate microwaves of very short wavelength (5 to 3 mm, that is to say at frequencies from 40 to 90 GHz), whose wavelength must lie in the range of the physical extent of the foreign bodies to be detected, with regard to their scattering behavior on foreign bodies and to use them for foreign body detection (U.S. Pat. No. 4,707,652, Lowitz, Philip Morris Inc., New York, 1985). Since the penetration of electromagnetic waves into a dielectric product is approximately of the order of the wavelength, owing to the microwave attenuation resulting from the material moisture, the behavior has only a limited effect, owing to the limited penetration depth. Furthermore, in particular not just foreign bodies act as scattering centers but also all density fluctuations in the normal product stream, such as cannot be avoided at all in typical fibrous material or bulk goods.
A transillumination method is also known, in which the product is located between a transmitting and a receiving antenna, with which foreign bodies are to be detected (WO 00/09983, Moshe, Malcam Ltd. Nazareth-Israel, 1998). However, this method has the disadvantage that, as is also the case in other transillumination methods, the received signal also depends very critically on other changes in the material stream. The method is therefore inaccurate and requires very complicated evaluation using neural networks or “fuzzy logic” (pp. 32/33 of the citation).
In a proposed method of the type mentioned at the beginning (WO 02/09539 A1), combination signals on the basis of the changes in the resonant frequency and the width of the resonance curve are calculated in order to detect foreign bodies.